1. Field of the Invention
The present invention relates to a motor assembly with coaxial shafts and more particularly to a motor assembly with coaxial shafts in which a plurality of rotary shafts are adapted to rotate on a common axis.
2. Description of the Related Art
Such type of motor assembly with coaxial shafts (hereinafter referred to simply as “motor assembly” as appropriate) is conventionally known in Patent Reference 1 (Japanese Patent No. 2587913). The motor assembly disclosed in Patent Reference 1 has a hollow rotation shaft and a solid rotation shaft inserted into the hollow rotation shaft and each of the rotation shafts is supported by plain bearings.
In the conventional motor assembly disclosed in Patent Reference 1, a motor adapted to rotate the hollow rotation shaft and another motor adapted to rotate the solid rotation shaft are individually provided and these two motors are coupled to each other in such a manner that a recess formed on a plain bearing of one of the two motors is fitted with a boss formed on a plain bearing of the other motor.
Plain bearings are used in the motor assembly disclosed in Patent Reference 1. However, in general, working accuracy for an inner circumferential surface and an outer circumferential surface of the plain bearings is at best several tens of micrometers and, therefore, in case of a stepping motor having an air gap of about 60 micrometers between a stator and a rotor, there is a possibility of contact between the stator and the rotor due to the deviation of the center.
Further, the motor assembly disclosed in Patent Reference 1, in which the boss and the recess formed on the respective plain bearings are fitted with each other to thereby couple together these two motors, has another problem in that oil exudes onto the surface of the plain bearings, that is, oil-impregnated sintered bearings, causing friction force between the boss and the recess to be lowered and thus decreasing coupling strength between the two motors. Even if an adhesive material is used for ensuring the coupling, adhesion force is lowered by the oil.
Another proposed solution is to use ball bearings instead of plain bearings. However, when a recess and a boss which are formed on outer or inner rings of the ball bearings, are fitted with each other to couple the motors, the ball bearings are subjected to excessive external force, which shortens the life of the bearings. In addition, the excessive external force imposed causes torque variation in the motors. Moreover, high working and assembling accuracy is required for forming the recess and the boss on the outer or inner rings of the ball bearings, which makes the production difficult and thus indicating a highly probable increase in costs.